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Chemistry · 2024 All years Show answers Download PDF PDF with answers
Roll No.
Q.P. Code 56/1/1

Chemistry (Theory)

CBSE Class XII Board Examination 2024

Time allowed: 3 hoursMaximum Marks: 70

General Instructions:

  1. The paper has 33 questions in five sections (A to E); all of them must be answered.
  2. Section A: Q1–16, multiple choice (including Assertion-Reason), 1 mark each.
  3. Section B: Q17–21, very short answer, 2 marks each.
  4. Section C: Q22–28, short answer, 3 marks each.
  5. Section D: Q29–30, case-based, 4 marks each.
  6. Section E: Q31–33, long answer, 5 marks each.
  7. There is no overall choice, but some questions in Sections B to E offer an internal choice (OR). Calculators are not allowed.

SECTION A

Q1–16 are multiple choice questions of 1 mark each (Q13–16 are Assertion-Reason).
1.
Among these first-row transition elements, which one should have the highest third ionisation enthalpy?
  1. (A) Iron (Z = 26)
  2. (B) Manganese (Z = 25)
  3. (C) Chromium (Z = 24)
  4. (D) Vanadium (Z = 23)
1
2.
Which of these compounds gives a ketone when oxidised with chromic anhydride (CrO₃)?
  1. (A) (CH₃)₂CH–CH₂OH
  2. (B) CH₃CH₂CH₂OH
  3. (C) (CH₃)₃C–OH
  4. (D) CH₃–CH₂–CH(OH)–CH₃
1
3.
A DNA nucleotide is made of three parts. Two of them are β-D-2-deoxyribose and a heterocyclic base. What is the third part?
  1. (A) Adenine
  2. (B) Phosphoric acid
  3. (C) Sulphuric acid
  4. (D) Uracil
1
4.
When a solute associates in a solvent, its van't Hoff factor i is:
  1. (A) always greater than one
  2. (B) negative
  3. (C) zero
  4. (D) always less than one
1
5.
For the reaction X + 2Y → P, which differential rate expression is correct?
  1. (A) \( \frac{2\,d[P]}{dt} = \frac{-d[Y]}{dt} \)
  2. (B) \( \frac{-d[P]}{dt} = \frac{-d[X]}{dt} \)
  3. (C) \( \frac{+d[X]}{dt} = \frac{-d[P]}{dt} \)
  4. (D) \( \frac{-2\,d[Y]}{dt} = \frac{+d[P]}{dt} \)
1
6.
Which compound undergoes SN1 reaction fastest?
  1. (A) 3-Bromocyclohexene (Br on the ring carbon next to the ring C=C, i.e. allylic)
  2. (B) (Bromomethyl)cyclohexane, C₆H₁₁–CH₂Br
  3. (C) Bromocyclohexane, C₆H₁₁–Br
  4. (D) Bromobenzene, C₆H₅–Br
1
7.
What is formed when acetic acid reacts with PCl₅?
  1. (A) Cl–CH₂–COCl
  2. (B) Cl–CH₂–COOH
  3. (C) CH₃–COCl
  4. (D) CCl₃–COOH
1
8.
Formation of a cyanohydrin from an aldehyde is an example of:
  1. (A) nucleophilic addition
  2. (B) electrophilic addition
  3. (C) nucleophilic substitution
  4. (D) electrophilic substitution
1
9.
In the Arrhenius equation, a graph of log k against 1/T is a straight line. Its slope and intercept are:
  1. (A) slope A/R, intercept Eₐ
  2. (B) slope A, intercept −Eₐ/R
  3. (C) slope −Eₐ/RT, intercept log A
  4. (D) slope −Eₐ/(2.303 R), intercept log A
1
10.
The reaction of an alkyl halide with a sodium alkoxide to give an ether is called:
  1. (A) Wurtz reaction
  2. (B) Reimer-Tiemann reaction
  3. (C) Williamson synthesis
  4. (D) Kolbe reaction
1
11.
Choose the correct order of ease of dehydration with conc. H₂SO₄:
  1. (A) (CH₃)₃C–OH > (CH₃)₂CH–OH > CH₃CH₂–OH
  2. (B) (CH₃)₂CH–OH > CH₃CH₂–OH > (CH₃)₃C–OH
  3. (C) CH₃CH₂–OH > (CH₃)₂CH–OH > (CH₃)₃C–OH
  4. (D) (CH₃)₂CH–OH > (CH₃)₃C–OH > CH₃CH₂–OH
1
12.
Which groups of glucose react with each other to give the cyclic hemiacetal (pyranose) form?
  1. (A) Aldehyde group and –OH at C-4
  2. (B) Aldehyde group and –OH at C-5
  3. (C) Ketone group and –OH at C-4
  4. (D) Ketone group and –OH at C-5
1
13.
Assertion (A): Adding NaCl to water lowers its freezing point.
Reason (R): NaCl dissociates in water.
  1. (A) Both A and R are true, and R is the correct explanation of A.
  2. (B) Both A and R are true, but R is not the correct explanation of A.
  3. (C) A is true, but R is false.
  4. (D) A is false, but R is true.
1
14.
Assertion (A): Zr and Hf are hard to separate.
Reason (R): Because of lanthanoid contraction, Zr and Hf have almost the same radii.
  1. (A) Both A and R are true, and R is the correct explanation of A.
  2. (B) Both A and R are true, but R is not the correct explanation of A.
  3. (C) A is true, but R is false.
  4. (D) A is false, but R is true.
1
15.
Assertion (A): Ethanoic acid has a lower pKₐ than Cl–CH₂–COOH.
Reason (R): Chlorine's electron-withdrawing (–I) effect increases the acidity of Cl–CH₂–COOH.
  1. (A) Both A and R are true, and R is the correct explanation of A.
  2. (B) Both A and R are true, but R is not the correct explanation of A.
  3. (C) A is true, but R is false.
  4. (D) A is false, but R is true.
1
16.
Assertion (A): Aniline is more basic than ammonia.
Reason (R): In aniline the lone pair on nitrogen is delocalised into the ring by resonance, so it is less available for protonation.
  1. (A) Both A and R are true, and R is the correct explanation of A.
  2. (B) Both A and R are true, but R is not the correct explanation of A.
  3. (C) A is true, but R is false.
  4. (D) A is false, but R is true.
1

SECTION B

Q17–21 are very short answer questions of 2 marks each.
17.
Find the electrode potential of an iron electrode dipped in 0.01 M Fe²⁺ solution at 298 K.
(E°(Fe²⁺/Fe) = −0.45 V; log 10 = 1)
2
18.
What is meant by molecularity of a reaction? Give one condition under which a bimolecular reaction can behave kinetically as first order.
2
19.
What is formed when D-glucose is treated with:
(a) HI (prolonged heating)
(b) conc. HNO₃
2
20.
Draw the major monohalo product in each reaction:
(i) Styrene (C₆H₅–CH=CH₂) + HBr →
(ii) Cyclohexene + Br₂ in UV light →
OR
Give reasons:
(i) Grignard reagents must be prepared in anhydrous conditions.
(ii) With aqueous KOH alkyl halides give alcohols, but with alcoholic KOH they give alkenes.
2
21.
Write the chemical equation when:
(a) butan-2-one is treated with Zn(Hg) and conc. HCl
(b) two molecules of benzaldehyde are treated with conc. NaOH
2

SECTION C

Q22–28 are short answer questions of 3 marks each.
22.
A conductivity cell filled with 0.05 M KCl shows a resistance of 100 Ω at 25 °C. The same cell filled with 0.02 M AgNO₃ shows 90 Ω. Find the conductivity and molar conductivity of the AgNO₃ solution.
(Conductivity of 0.05 M KCl = 1.35 × 10⁻² Ω⁻¹ cm⁻¹)
3
23.
For 2NO(g) + Br₂(g) → 2NOBr(g), the initial rates measured were:
Expt 1: [NO] = 0.05, [Br₂] = 0.05 mol L⁻¹, rate = 1.0 × 10⁻³ mol L⁻¹ s⁻¹
Expt 2: [NO] = 0.05, [Br₂] = 0.15, rate = 3.0 × 10⁻³
Expt 3: [NO] = 0.15, [Br₂] = 0.05, rate = 9.0 × 10⁻³
(a) Find the order with respect to NO and Br₂.
(b) Find the rate constant k.
(c) Find the rate when [NO] = 0.4 M and [Br₂] = 0.2 M.
3
24.
(a) Write the formula of potassium tetrahydroxidozincate(II).
(b) Put these in increasing order of conductivity of their solutions: [Cr(NH₃)₅Cl]Cl₂, [Cr(NH₃)₃Cl₃], [Cr(NH₃)₆]Cl₃
(c) Name the isomerism shown by: (i) [Co(NH₃)₅NO₂]²⁺ (ii) [Co(en)₃]Cl₃
3
25.
(a) Which one is an allylic halide: (i) CH₃–CH=CH–Br or (ii) CH₂=CH–CH(Br)–CH₃?
(b) Which is more reactive towards nucleophilic substitution – chlorobenzene or 2,4,6-trinitrochlorobenzene – and why?
(c) Which isomer of C₄H₉Cl has the lowest boiling point?
3
26.
(a) Write the mechanism of: CH₃CH₂OH → (H⁺, 443 K) → CH₂=CH₂
(b) Write the main product:
(i) CH₃–CH₂–CH=CH₂ treated with (a) B₂H₆ and then (b) H₂O₂/OH⁻
(ii) Phenol treated with (a) aq. NaOH and then (b) CO₂, H⁺
3
27.
Answer any three:
(a) What is a peptide linkage?
(b) What bonds hold the two strands of the DNA double helix together?
(c) Which of these is a polysaccharide: sucrose, glucose, starch, fructose?
(d) Give one example each of a water-soluble and a fat-soluble vitamin.
3
28.
Compound A (C₆H₁₂O₂) is reduced by LiAlH₄ to two compounds B and C. Oxidation of B with PCC gives D. D with dilute NaOH followed by heating gives E. Catalytic hydrogenation of E gives C. Further oxidation of D gives F, a monobasic acid of molar mass 60. Identify A to F.
3

SECTION D

Q29–30 are case-based questions of 4 marks each (choice in part (c)).
29.
Batteries and fuel cells are practical galvanic cells. A useful battery must be light, compact and give a nearly constant voltage. Primary batteries work only once, while secondary batteries can be recharged. Fuel cells convert the energy of combustion of a fuel directly into electricity (one was used in the Apollo space programme).
(a) How do primary batteries differ from secondary batteries? (1)
(b) A mercury cell gives 1.35 V that stays constant throughout its life. Why? (1)
(c) Write the reactions that take place when a lead storage battery is recharged. (2)
OR
(c)
Write two advantages of fuel cells over other galvanic cells.
4
30.
Valence bond theory explains formation, magnetism and shape of complexes, while crystal field theory looks at how ligands (taken as point charges) split the degenerate d-orbitals of the metal. The splitting gives different electron arrangements in strong and weak fields, and d–d transitions explain the colour of complexes. Complexes are used in metallurgy, analysis and medicine.
(a) What is crystal field splitting energy? (1)
(b) Explain the violet colour of [Ti(H₂O)₆]³⁺ using crystal field theory. (1)
(c) Explain why [Cr(NH₃)₆]³⁺ is paramagnetic but [Ni(CN)₄]²⁻ is diamagnetic. [Cr = 24, Ni = 28] (2)
OR
(c)
Explain why [Fe(CN)₆]³⁻ is an inner orbital complex while [Fe(H₂O)₆]³⁺ is an outer orbital complex. [Fe = 26]
4

SECTION E

Q31–33 are long answer questions of 5 marks each.
31.
(i) At the same temperature CO₂ is more soluble in water than O₂. Which gas has the higher Henry's constant K_H, and why?
(ii) What happens to the size of red blood cells placed in an aqueous solution containing more than 0.9% (mass/volume) NaCl?
(iii) A 1 molal aqueous solution of electrolyte A₂B₃ is 60% ionised. Find the boiling point of the solution. (K_b of water = 0.52 K kg mol⁻¹)
OR
(i) At 25 °C the vapour pressures of pure A and B are 75 mm Hg and 25 mm Hg. A and B are mixed so that the mole fraction of A in the liquid is 0.4. Find the mole fraction of B in the vapour.
(ii) Define a colligative property. Which colligative property is preferred for finding the molar mass of macromolecules?
(iii) Why are equimolar solutions of NaCl and glucose not isotonic?
5
32.
Answer any five:
(a) Why is N,N-diethylbenzenesulphonamide insoluble in alkali?
(b) Why does aniline not undergo Friedel-Crafts reaction?
(c) Give a simple chemical test to tell methylamine from aniline.
(d) Write the reaction(s) of Gabriel phthalimide synthesis.
(e) How will you convert aniline into p-bromoaniline?
(f) Complete: benzenediazonium chloride treated with (i) HBF₄, then (ii) NaNO₂/Cu, heat →
(g) Benzoic acid is heated with NH₃ to give A; A with Br₂ + NaOH gives B. Write A and B.
5
33.
(i) Account for:
(1) Zn, Cd and Hg have low melting and boiling points.
(2) Both are d⁴ species, yet Cr²⁺ is a strong reducing agent while Mn³⁺ is a strong oxidising agent.
(3) E° for Cu²⁺/Cu is +0.34 V (positive).
(ii) Complete and balance:
(1) KMnO₄ → (heat)
(2) Cr₂O₇²⁻ + 6I⁻ + 14H⁺ →
OR
(i) Which is more stable in aqueous solution, Cu₂Cl₂ or CuCl₂, and why?
(ii) Write the general electronic configuration of f-block elements.
(iii) Which of Sc³⁺, Fe³⁺, Zn²⁺ is coloured in aqueous solution, and why? [Sc = 21, Fe = 26, Zn = 30]
(iv) How is potassium dichromate obtained from sodium chromate?
(v) Why do transition metals and their compounds act as catalysts?
5
— END —
Questions restated from the CBSE Class XII Board Examination 2024 (set 56/1/1) for practice. Reading passages and map items are described, not reproduced.